Multiple beam lasing systems can be used in a variety of applications such as printing systems wherein rotating polygonal mirrors are used to scan the light beams across a receptive surface. The use of multiple laser beams in a printing system provides the capability of producing more than one line of information at a time, thus providing high speed printing with relatively slow movement of the rotating polygonal mirror. Multiple beam devices provide other capabilities as well; that is, the multiple beams can be used to alter the shape of the effective writing spot by modulating the spots within the spot group or they can be used to modulate the amount of light provided at each picture element (pel) position.
While the use of multiple beam lasing systems have significant advantages over a single beam lasing system, it has been found difficult to provide a system in which all of the beams are in focus at the same plane and in which all of the beams are coordinated to write pels at identical locations in the scan lines or in-line with one another on adjacent lines. These problems are in addition to various other problems which occur in conventional single beam scanning system designs, including F-.theta., flat-field, tilt, and diffraction limited issues.
The F-.theta. problem is caused by using a rotating polygonal mirror, wherein the lasing beam is scanned across the image plane by the facets of the polygon. As the polygon rotates the angle of scan, .theta., changes linearly in time. The linearity problem causes pels at the edges of the scan to have a larger spacing than the pels at the center of the scan. Consequently, placement of pels is a function of the scanning angle, .theta.. One way to compensate for this problem is to provide a negative power scanning lens group and a positive power scanning lens group to create distortion in the scan so that the pels are more evenly spaced at the image plane across the entire scan.
The flat-field problem is related to proper focus at the image plane where the image plane lies in a flat surface. The problem results from the fact that it is a shorter distance to a flat image plane at the center of the scan than it is at the edges of the scan. Therefore, if focus is provided at the center of the scan the pels may be slightly out of focus at the edges, resulting in a larger edge spot. In order to maintain a small and compact system with relatively short focal length, the flat-field problem is approached by utilizing negative and positive power scanning lens groups, and adjusting the distance between the two lens groups in order to achieve proper focus at the image plane across the entire scan.
Another problem of the conventional single beam lens system is the so-called tilt problem, which is a result of pyramidal facet errors caused by manufacturing imperfections in producing the facets of the rotating polygonal mirror. Tilt errors are minimized through the use of a cylindrical lens together with an anamorphic scan lens set. The cylindrical lens focuses the beam in one dimension at the facet. In that manner, the beam will appear as a line across the facet and will be less subject to tilt errors. After reflection from the facet, the light beam then passes into an anamorphic scan lens set in order to reproduce the original shape of the light beam; that is, to focus the light at the image plane in a slightly elliptical shape.
Still another problem of the single beam lens system relates to the different amount that a beam is refracted depending on whether the beam enters the lens at the center or enters the lens at the edge. This results in the focal point changing for those beams passing through the edge of the lens relative to beams passing through the thicker mid-portion of the lens. In order to minimize the size and expense of a lens system, a complex lens is used which is of diffraction limited design; that is, designed to keep the spot size the same across the scan regardless of whether the light rays pass through the edge of the lens or through the middle of the lens.
The inventors discovered that when a multiple beam scanning lens system was designed to provide F-.theta. correction, flat-field correction, tilt correction, and diffraction limited corrections, all of the optical corrections needed for a single beam system, that system still did not operate properly. The problems were that the multiple beams were not all in focus and that the beams were refracted through the lens system a different amount such that they were separated at the image plane. As a result, the pel size was not the same from beam to beam, and when the beams were used to write different lines of information at the image plane, the beams were separated and did not line up properly with pels placed one directly under the other. The inventors came to understand that these effects were caused by the fact that in a single beam lasing system there is only one wavelength to consider, while in a multiple beam lasing system of four (4) lasing sources, there are four (4) wavelengths of light to consider. As a result, in addition to all of the monochromatic corrections previously discussed, the system must be color corrected in order to operate properly. Recognizing that problems of different wavelengths are associated with the refraction of each beam a different amount, lens glasses were chosen that were less dependent on wavelength. In addition, other approaches were taken such as reoptimizing the design with heavy weight on chromatic performance, focussing the collimating lens to produce a converging beam into the scan set, and raising the index of refraction of the scan set elements. Despite these efforts, satisfactory performance was not achieved.
It is, therefore, an object of this invention to produce an optical system for use in a multiple beam scanning printhead apparatus which provides both monochromatic and chromatic compensation.
It is an object of this invention to maintain light from all beams in a multiple beam printhead in focus at the image plane.
It is another object of this invention to compensate for the different refraction of different wavelength light such that pels will line up properly at the image plane.
It is an object of this invention to provide a multiple beam printhead such that proper monochromatic and chromatic operation is achieved while utilizing lasing sources of significantly different wavelength.